Hypothyroidism: an early sign of vitamin A deficiency in chickens.

The effect of a marginal vitamin A deficiency on thyroid function was investigated in chicks gaining weight. This study examined the hypothesis that hypothyroidism is one of the earliest manifestations of hypovitaminosis A in chicks. Day-old Leghorn cockerels were fed vitamin A-adequate and deficient diets. Blood vitamin A levels were reduced in the deficient chicks by 3 weeks of age. Body weight gains were less in the 5-, 6-, 7- and 8-week-old A-deficient chicks than in controls. Compared to controls, vitamin A-deficient chicks were characterized by thyroid follicle hypertrophy by 4 weeks, heavier thyroids as a percent of body weight at 5 weeks, less serum thyroxine (T4) at 7 weeks and lower serum triiodothyronine (T3) and T4 at 8 weeks. These results indicate that one of the earliest signs of vitamin A deficiency in chicks is hypothyroidism.